1. Field of the Invention
This invention relates to calcium hydroxide particles having a moisture content of less than 2 weight % and a total desorption pore volume of at least 0.1 cm.sup.3 /g.
2. Description of the Related Art
In order to achieve a good collecting of gaseous effluents (SO.sub.2, HCl, . . . ) by means of calcium hydroxide particles (in other words a good chemical reactivity), persons skilled in the art have prepared Ca(OH).sub.2 particles whose texture has been modified in order to show large diameter pores in order to ensure a good internal diffusion. Preferably, the texture is also modified in order to increase its intrinsic activity, in other words in order to ensure a large specific surface area.
Various methods for the preparation of calcium hydroxide particles of particular texture are known.
Thus, by the alcohol method, method in which quicklime is slaked in the presence of a large quantity of alcohol, a hydrated lime is prepared containing alcohol (the complete elimination of this latter being impossible) characterised by a small particle size distribution (less than 20 .mu.m), a large specific surface area (+30 m.sup.2 /g) and a low water content. This method necessitates an expensive installation, because it is necessary to recycle to a maximum the used alcohol.
According to an uneconomic method, calcium hydroxide particles are apparently obtained by drying a lime milk obtained by slaking of CaO with a weight ratio water/lime greater than 2. This uneconomic method, in view of the quantity of water to be evaporated, only permits the preparation of Ca(OH).sub.2 particles of small particle size distribution (particle size distribution mainly less than 10 .mu.m). Furthermore, the reactivity to CO.sub.2 of lime in the form of milk is significant. This slaked lime after drying is in the form of particles mainly showing pores with a diameter of less than 100 .ANG. (hypothesis of a cylindrical pore geometry).
Lastly, from the document PCT/BE 91/00082 a method is known from which the lime is slaked in the presence of particular additives, such as ethylene glycol, . . . .
The hydrated lime obtained by this method is in the form of particles of uncontrolled particle size distribution, having a residual moisture that is difficult to control capable of leading to problems in use, and having a large specific surface area (greater than 35 m.sup.2 /g). This lime has a pore volume consisting of pores with a diameter ranging from 100 to 400 .ANG., less than 0.06 cm.sup.3 /g (the diameter of the pores being calculated with the hypothesis of a cylindrical geometry, BJH method).
From the document U.S. Pat. No. 2,894,820 methods for the preparation of hydrated lime particles are also known. According to a first method described in this document, quicklime is slaked in such a way as to produce a dry calcium hydroxide, and this dry calcium hydroxide is subjected to a classification. According to another method, quicklime is slaked in such a way as to produce a moist calcium hydroxide, the aforementioned calcium hydroxide is dried and this calcium hydroxide is subjected to a classification. By these methods, according to this document calcium hydroxides are obtained that have BET specific surface areas of less than 25 m.sup.2 /g. From examples 1 and 2 of this document, it appears that the agglomerated particles obtained by this dry method or by the moist method have similar characteristics.
This document in no way specifies the type of quicklime to be used, nor the importance of using a particular moist hydration method, nor the precautions advantageously to be taken at the time of the drying stage of the moist calcium hydroxide, in order to obtain a calcium hydroxide having a nitrogen desorption pore volume consisting of pores with a diameter ranging from 100 to 400 .ANG., calculated according to the BJH method on the hypothesis of a cylindrical pore geometry, greater than 0.06 cm.sup.3 /g.
In the remainder of this description, by pore diameter is understood the diameter calculated according to the BJH method, with the hypothesis of a cylindrical pore geometry.